The present invention relates to a stepwise drive mechanism of the type comprising endless conveying members, such as metal chains, etc., wherein the said members are parallel to one another, which transport a thermoplastic film in which receptacles are formed, which traverse a machine for processing food and/or pharmaceutical products and which are driven synchronously by driving wheels, each of which is in contact with a corresponding conveying member along a contact arc, whereby an elementary angular displacement of the said driving wheel corresponds to a feed step of the conveying member which also passes on a return wheel.
Numerous installations are known wherein one or more conveying members are moved stepwise to bring the product or products which they convey successively to one or more fixed working stations. Such an installation is, for example, a machine for processing food and/or pharmaceutical products as described in French Pat. 2,028,765 wherein a thermoplastic film is successively brought by means of two lateral conveying and guidance members to a film heating station, a station for shaping the receptacles in the film, a station for filling the shaped receptacles, a station for welding a sheet to cover the filled receptacles and a cutting station for the filled and sealed receptacles. Each film conveying and guidance member comprises an endless chain extending along the machine and a system of clips gripping the corresponding edge of the film and mounted on the links of the said chain. The chains are driven by toothed wheels whereof one elementary angular displacement corresponds to one film feed step and passing over return wheels. The toothed wheels are themselves driven synchronously e.g. by means of a Maltese cross device.
The driving of a chain conveying member by toothed wheels has the disadvantage of not giving a strictly constant feed step to the conveying device. In the case of the above-mentioned processing machine, the different working stations are positioned at intervals whose length is a multiple of the feed step of the thermoplastic film conveying device and an irregularity of this feed step leads to a poor positioning of the thermoplastic film and of the receptacles formed therein at the working station. The irregularities of the feed step are mainly due to faults in the chains. The causes of these faults are on the one hand the impossibility of manufacturing and assembling the links forming the chains in an absolutely regular manner and on the other the wear and elongation of the chains under tension, which leads to the links becoming out of true. As in addition these faults are not identical on any two chains they end up to creating variations in the length of the feed step leading to positioning variations at the working stations of the order of a few millimetres for a machine having a length of several meters. Another cause of the irregularity in the feed step is that the cooled thermoplastic film after passing through the heating and shaping stations tends to retract and come closer to the links of the chains via clips carried by these links and which grip the edges of the film. To take account of these irregularities in the feed step and permit an operation of the machine without too frequent adjustments, it is necessary to provide large tolerances in the positioning of the thermoplastic film at the different stations, particularly at the welding and cutting stations, while the receptacles are given sufficiently wide flanges to ensure a satisfactory welding and cutting. The consumption of thermoplastic material is increased and the positioning variations between the receptacles and the sheet provided for covering the same and which is printed, impair the appearance of the sealed and cut receptacles.